Finger Bone Points to New Branch of Humanity

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A finger bone from Siberia now reveals a previously unknown group
of ancient humans once existed there, one neither like us nor
Neanderthals.

Bizarrely, the DNA from these extinct Siberians seems unusually
similar to that of Pacific Islanders from tropical Melanesia.

The 30,000-year-old fossil was found in Denisova Cave in southern
Siberia in 2008, a bone fragment that likely came from a
fingertip of a young girl. [ Image
of finger fragment ] It was discovered along with microblades
(small stone blades used as tools), body ornaments of polished
stone, and
a molar shaped very differently from that of Neanderthals and
modern humans, resembling that of much older human species, such
as Homo habilis and Homo erectus. (The tooth
and the finger bone apparently came from different members of the
same population.)

After an international team of researchers sequenced the DNA from
40 milligrams of bone they removed from the fossil, they found
the "Denisovan" (deh-NEESE-so-van) shared a common origin with
Neanderthals but was genetically distinct, apparently descending
from the same ancestral population of
Neanderthals that had separated earlier from the ancestors of
modern humans.

"It amazed me that we found this other extinct group of humans,"
evolutionary geneticist Svante Pääbo at the Max Planck Institute
for Evolutionary Anthropology at Leipzig, Germany, told
LiveScience. "When we got this little finger bone from Siberia, I
was totally expecting it to be either Neanderthal or modern
human. When it was something else, that was totally surprising
and shocking to me."

More interbreeding

Surprisingly, their analysis found that genetic material from
this sister group of Neanderthals matched 4 to 6 percent of the
genomes of some modern Melanesian populations. This suggests
interbreeding took place between Denisovans and the ancestors of
Melanesians, just as Neanderthals appear to have interbred with the
ancestors of all modern-day non-Africans.

"Instead of the clean story we used to have of modern humans
migrating out of Africa and replacing Neanderthals, we now see
these very intertwined story lines with more players and more
interactions than we knew of before," said researcher Richard
Green of the University of California, Santa Cruz.

The fact that this extinct branch of the human family tree was
discovered in Siberia but contributed gene sequences to modern
humans in Southeast Asia suggests it might have been widespread
in Asia during the late Stone Age, said researcher David Reich,
the evolutionary geneticist at Harvard Medical School who led the
new population genetic analysis.

It remains uncertain whether this genetic material might have
persisted in Melanesians because it provided an evolutionary edge
of some sort. "We have a hard enough time learning what effects
gene sequences might have when it comes to the genomes of modern
humans, such as disease susceptibility — to do that with an
archaic group is even harder," Reich told LiveScience.

When we met the Denisovans

These findings are adding to the complex picture of the evolutionary history of modern humans and our
extinct relatives that has recently emerged, one where
interbreeding has left its legacy in our DNA.

The researchers suggest an ancestral group left Africa between
300,000 and 400,000 years ago and quickly diverged, with one
branch becoming the Neanderthals who spread into Europe and the
other branch moving east and becoming Denisovans. When modern
humans left Africa roughly 70,000 to 80,000 years ago, they first
encountered the Neanderthals, with remnants of their DNA making
up 1 to 4 percent of the genomes of all non-Africans. Another
group of modern humans later came in contact with Denisovans.

"This study fills in some of the details, but we would like to
know much more about the Denisovans and their interactions with
human populations," Green said. "And you have to wonder if there
were other populations that remain to be discovered. Is there a
fourth player in this story?"

The researchers studiously avoid calling the Denisovans a new
species or subspecies. In fact, it remains highly contentious as
to whether the Neanderthals were another species altogether or
were a subspecies of our species. A species is a group different
enough from other groups as to be considered separate, and whose
members can and do interbreed — although research has recently
shown that Neanderthals (and now Denisovans) shared genes with
us, so it remains an open question as to how different they were.
Neanderthals and Denisovans are both called humans, however, just
as all members of the genus Homo are — the controversy
is over whether they should be lumped together with us
anatomically modern humans or not.

Future research can investigate whether the Denisovan or
Neanderthal remnants seen in modern humans provide any
evolutionary advantages, Reich added.

"Maybe this is the future — reconstructing extinct relatives
based not on what stone tools they made, but on their whole
genomes from only little pieces of bone," Pääbo said.

The scientists detailed their findings in the Dec. 23 issue of
the journal Nature.